/Engine/Source/Runtime/Engine/Public/Animation/AttributesRuntime.h
Add any new or accumulate with existing custom attributes
static void AccumulateAttributes ( const [FStackAttributeContainer](API\Runtime\Engine\Animation\FStackAttributeContainer) & SourceAttributes, [FStackAttributeContainer](API\Runtime\Engine\Animation\FStackAttributeContainer) & OutAttributes, float Weight, EAdditiveAnimationType AdditiveType )